1. Field of the Invention
This invention relates generally to the field of packaging, and more specifically to the field of plastic containers, such as those plastic containers that have size, shape and functionality that permits them to serve as a replacement for conventional metal cans.
2. Description of the Related Technology
The use of metal cans to package products such as foodstuffs is well over a century old. Conventional metal cans are either of a two-piece configuration, in which a lid is secured to a can body having an integral bottom portion using a double seaming process, or of a three-piece configuration in which a lid and a bottom member are respectively secured to opposite open ends of a substantially cylindrical can body.
Plastic cans have been proposed as a substitute for conventional metal cans, but to date they have not achieved any significant commercial success. The use of plastic to fabricate a can body offers a number of potential advantages, such as lower energy costs during both the manufacturing and recycling stages, better formability and less susceptibility to denting during handling. Dented metal cans present potential health risks, such as increased susceptibility to contamination that can lead to conditions such as botulism.
In addition, a can that is fabricated out of food grade plastic would not require potentially harmful coatings of such materials as Bisphenol A (BPA). However, plastic lacks the inherent strength of metals such as steel and aluminum. It also tends to soften at much lower temperatures than steel and aluminum.
Plastic containers such as plastic cans accordingly may lack the column strength that is necessary to avoid deformation of the sidewall of the container when a number of containers or palettes of containers are stacked during transportation or in packaging or retail facilities. While it is possible to increase the strength of a plastic container by increasing the thickness of the sidewall, doing so also increases manufacturing costs by increasing the amount of plastic material that is required. Lightweighting is an important consideration in the design of plastic containers, including plastic cans, because plastic material tends to be relatively expensive.
Many plastic containers such as plastic cans also typically lack the requisite circumferential or hoop strength that is required to avoid excessive deformation when the contents of the container becomes pressurized. Certain products, particularly food, require sterilization during the packaging process in order to inhibit the growth of bacteria.
The most common commercial procedure for heat sterilizing canned foods is a retort process in which filled but unsterilized sealed cans are placed in a retort chamber that is injected with steam and held at a predetermined elevated temperature (typically between about 210° F. to about 260° F.) for a predetermined period of time. Conventional plastic cans have been considered unsuitable for packaging applications in which heat sterilization is required, because the heat and pressurization that is inherent to such processes has the tendency to cause irreversible damage and deformation to the sidewall of the plastic can. Positive pressurization is typically developed within the container during the retort process as a result of the expansion that occurs when the contents of the container are heated. As the container cools after the retort process, negative pressurization can also be an issue, particularly in instances where there is a large amount of headspace within the container.
Metal cans are also commonly used to package pressurized beverages such as beer and soft drinks. In addition, other beverages that are not carbonated may develop a positive pressure with respect to ambient atmospheric conditions when the container is heated or transported to higher altitudes. Conventional extrusion blow molded plastic cans have been considered unsuitable for use in the packaging of such beverages.
A need exists for a plastic container that has sufficient column strength and hoop strength to replace a conventional metal can, and that has sufficient rigidity and stability under elevated pressures and temperatures to permit heat sterilization without experiencing excessive deformation. A need further exists for a plastic container that has sufficient strength to resist internal pressurization, so that it could be used to package carbonated beverages and the like. Moreover, a need exists for a plastic container that has sufficient strength to resist negative pressurization that may develop within the container after heat sterilization. In addition, a need exists for a method of producing a heat sterilized packaged product that utilizes a plastic container.